1. Field of the Invention
The present invention relates to an apparatus and a method for controlling the ignition timing of a cylinder-injection and spark ignition type internal combustion engine to prevent the engine from knocking during its transient operation and ensure both improvement of the engine output and reduction of the fuel consumption.
2. Description of the Related Art
Internal combustion engines mounted in automobiles are expected to be designed so that they decrease harmful components in the exhaust gas therefrom and consume less fuel. Accordingly, there have recently been proposed cylinder-injection and spark ignition type internal combustion engines (the engine of this type is hereinafter referred to as DI engine) in which fuel is injected directly into their combustion chamber, in place of conventional spark ignition type internal combustion engines, i.e., conventional gasoline engines, in which fuel is injected into their intake port or intake manifold.
A DI engine is provided with a fuel injection valve for directly injecting the fuel into a combustion chamber thereof. More specifically, a cavity is formed on the top of a piston fitted in the cylinder, and the fuel injection valve directly injects the fuel into the combustion chamber or the cavity in a final stage of the compression stroke when the DI engine is in low-load operation. In the cavity, the injected fuel forms an air-fuel mixture with an air-fuel ratio near the theoretical air-fuel ratio around a spark plug. Even though the average air-fuel mixture in the entire combustion chamber is lean, therefore, the fuel can be securely ignited by the spark plug, since the air-fuel mixture around the spark plug has the air-fuel ratio near the theoretical air-fuel ratio. As a result, CO and HC contained in the exhaust gas from the DI engine are reduced, and the fuel consumption of the DI engine is lessened substantially during idle operation or low-load operation of the DI engine.
On the other hand, the fuel injection valve directly injects the fuel into the combustion chamber in an initial stage of the intake stroke during high-load operation of the DI engine. The injected fuel uniformly makes a stoichiometric or relatively rich air-fuel mixture in the combustion chamber so that the DI engine output is increased.
Unlike the conventional gasoline engines, the DI engine therefore is free from a delay in fuel transportation, since the fuel is injected directly into the cylinder or the combustion chamber, as described above. In a conventional gasoline engine, fuel is injected into its intake port or intake manifold, so that it is impossible to avoid the delay in fuel transportation, that is, a lag between the time of fuel injection and the time when the fuel is practically introduced into the combustion chamber. If the injection quantity of the fuel is increased or decreased in the DI engine, such increase or decrease is therefore reflected immediately in acceleration or deceleration of the DI engine, so that the engine response is substantially improved.
The output and fuel consumption of the conventional gasoline engine depend greatly on the ignition timing of the spark plug. In other words, the ignition timing is an important factor that determines the engine output and fuel consumption. More specifically, if an air-fuel mixture is burned under the same conditions except the ignition timing, the engine output or torque obtained by the combustion of the fuel is substantially influenced by the ignition timing. In this case, the ignition timing for making the conventional gasoline engine generate a maximum output thereof exists before the top dead center of the piston, and the advance of the ignition timing is referred to as MBT (minimum spark advance for the best torque). If the advance of the ignition timing is increased or decreased from the MBT, the combustion pressure in the cylinder cannot be utilized effectively. Thus, the engine output decreases, while the fuel consumption increases.
Even though the advance of the ignition timing is set to the MBT, however, the conventional gasoline engine may knock due to the octane value of the fuel, engine loads, etc. In most operation regions of the conventional gasoline engine, therefore, the ignition timing is generally adjusted to a basic ignition timing that is reduced or retarded by a given margin compared to the MBT.
When the conventional gasoline engine is in a stable operation region, the basic ignition timing is normally set according to a map based on the engine speed, loads, etc. as parameters. When the conventional gasoline engine is in a transient operation region, however, the ignition timing is controlled so as to be further retarded or advanced compared to the basic ignition timing obtained from the map. Disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-223647, for example, teaches a technique for temporarily retarding the ignition timing behind the basic ignition timing and then advancing it ahead of the basic ignition timing for a given period. By this technique, a conventional gasoline engine can be prevented from knocking, and its output can be improved. More specifically, when accelerative operation of the conventional gasoline engine is started, the injection quantity of the fuel is increased, whereupon the quantity of the fuel that adheres to the inner wall surface of the intake port or the intake manifold of the conventional gasoline engine also increases. Immediately after fuel injection, therefore, low-octane or high-volatility components of the fuel are introduced more than high-octane components of the fuel into the combustion chamber. If the ignition timing is retarded behind the basic ignition timing at this point of time, however, the conventional gasoline engine can be prevented from knocking. On the other hand, the wall temperature of the combustion chamber never rises suddenly when the ignition timing is retarded behind the basic ignition timing. If the ignition timing is advanced toward the MBT after the start of the engine acceleration, therefore, the engine output can be improved without causing the conventional gasoline engine to knock.
In the DI engine, the fuel is directly injected into the cylinder or the combustion chamber, so that there is no possibility of more low-octane components than high-octane components flowing into the combustion chamber. If the aforementioned ignition timing control for the conventional gasoline engine is applied to the DI engine, therefore, the engine output is unfavorably lowered owing to the retard of the ignition timing, and besides, the fuel consumption inevitably increases when the accelerative operation of the engine is started.